In this study, we will determine the role of a novel protein MPZL3 in skin keratinocyte stem cell (SC) maintenance, using the rough coat (rc) mice as a model system. The MPZL3 protein was first described by our laboratory in 2007. This protein is predicted to be a transmembrane protein, and may function as a cell adhesion molecule. We have demonstrated its localization to the plasma membrane in various epithelial cells, including skin keratinocytes of the epidermis and hair follicles (HFs). Within the HFs, the SCs residing in the bulge region are responsible for the cyclic growth of the HF, as well as epidermal wound healing. The rc mice contain a spontaneous mutation in Mpzl3, and homozygous rc/rc mice develop severe abnormalities of the skin, including cyclic and progressive hair loss, sebaceous gland hypertrophy, and spontaneous and non- healing skin lesions. These abnormalities may suggest diminished HF SCs in the bulge, with a concomitant increase in sebaceous lineage cells. Whereas MPZL3 expression is widespread in various epithelial cells, one function of MPZL3 may be to maintain the bulge SCs. Our long-term objective is to understand the role of MPZL3 in epidermal differentiation and function, and how dysregulation of MPZL3 may be involved in skin diseases. We hypothesize that MPZL3 functions as a cell adhesion protein important for maintaining the SC population in the HF bulge. To test this hypothesis, we will investigate whether the rc mutation leads to diminished SCs in the HF bulge in the rc/rc mouse skin. We will examine SC marker expression in the rc/rc skin, and grow isolated SCs in culture to determine their proliferative potential. We will also use in vitro analysis to determine the role of MPZL3 in keratinocyte adhesion and migration. Our study will not only elucidate the role of the novel protein MPZL3 in keratinocyte function, it will also shed light on how the rc phenotype develops. Maintaining SCs is crucial for tissue homeostasis and regeneration, and a better understanding of the underlying mechanisms will have great impact on regenerative medicine. PUBLIC HEALTH RELEVANCE: Mpzl3 is a novel gene first described by our laboratory two years ago, and mice with a single mutation in this gene develop severe skin and hair abnormalities. In this study, we will determine the role of this MPZL3 protein in skin keratinocyte function and in maintaining the skin stem cells. Through this study, we may gain important insight on wound repair and tissue regeneration.